Android ThreadUtils: Make the class public for access outside org.webrtc

Also make the class non-final. We shouldn't use non-final classes, because then we can't mock them.

R=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/1470973002 .

Cr-Commit-Position: refs/heads/master@{#10757}

talk/app/webrtc/java/android/org/webrtc/ThreadUtils.java

@@ -34,7 +34,7 @@ import java.util.concurrent.Callable;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
-final class ThreadUtils {
+public class ThreadUtils {
   /**
    * Utility class to be used for checking that a method is called on the correct thread.
    */